This invention concerns automatically resetting torque limiting clutches, commonly referred to as limiters and more particularly automatically resetting torque limiters which can disconnect drive on overload.
Torque limiters are designed to release a driving connection when an excessive load is experienced. Such limiters are typically used in the context of a drive for various types of machinery subject to jams and other overloads in order to not damage the driving motor or the machinery. In most instances, the release of the torque limiter is noticed soon after it occurs since the machinery ceases to operate, and the need to reset the torque limiter input becomes apparent.
Automatic resetting torque limiting clutches have been in existence for many years, typically of a friction clutch or ball detent type. The friction type will release or slip at a preset overload torque value, and will reengage when the overload is removed. The disadvantage of this arrangement is that repeated heating of the torque limiter friction linings (as heat is generated by the slipping) causes the clutch capacity to fade, as the higher lining temperatures reduces the coefficient of friction, until the torque limiter slips continuously and destroys itself.
Another long known torque limiter type is the ball-detent reset torque limiter, which uses spring forces to push balls into drill point cavities in the other member, with the geometry thereof establishing forces and angles to produce a retraction of said balls to create a release at a preset torque level. The torque limiter will reengage when the torque demand falls somewhere below the release torque. The disadvantage of this device is the sudden changes in the acceleration of the connected components, which produces stresses in the material which exceeds the elastic limits of the components when running, disengaged, or when reengaging, which in turn produces deformations which greatly reduce the torque limiter service life.
There have also been developed further refinement of such torque limiters in which smoothly curved engagement surfaces define the cavities in an attempt to reduce the stresses occasioned by engagement and reengagement.
In some torque limiter installations, relatively high speeds of the input is involved, commonly as high as motor speed, when there is no reduction gearing interposed between the motor and the input to the torque limiter lowering the rotational speed of the input. In those instances of high speed torque limiter release, the overrunning components would be subjected to severe shock loads as a result of being driven at these high rotational speeds, which can be of the order of 1800 rpm or greater.
This could cause deformation of the mating torque limiter components, leading to failure or to changes in the set and reset torque levels due to deformations of the follower elements or engagement surfaces.
These applications create severe demands in cases where access to the torque limiters is very difficult or impossible as when the machine is located in tight spaces such as in tunneling and mining operations. In these applications, a torque limiter will likely be operated in the disengaged mode for extended time periods, and complete failures or significant degrading of performance would be very undesirable and costly.
A satisfactory torque limiter for such high speed limited access applications has not yet been developed.
Those working in the torque limiter field and on resettable torque limiters of the type described have heretofore long recognized that the drive surfaces which act to interrupt the drive through the limiter by causing drive elements under excessively high loads to be moved to a released position should be smoothly curved to reduce shocks when engaging and disengaging these elements. See U.S. Pat. No. 2,501,648 for a discussion of these matters in the context of an automatically resetting torque limiter.
In the field of cams design in a machine where there is a particular designed for output motion there has long been recognition that simply smoothly curving these surfaces is not sufficient to avoid shocks and early failures.
However, in the field of automatic resetting torque limiters, it has not heretofore been recognized that simply smoothly curving the engagement surfaces which are engaged by drive elements is not sufficient to eliminate noise and impact wear of the engagement surfaces and elements when continuously operating in the released condition at high speeds, i.e. around 1800 rpm or higher. In fact, despite a long felt need for such a torque limiter, no automatically reset torque limiter has been produced which can operate successfully under those conditions.
It is the object of the present invention to provide an automatically resettable torque limiter which is capable of operating in its released mode at high speeds for extended periods without damaging the mechanism such as to result in a failure or in a progressive change in the release and resetting torque levels.
It is a further object to provide such an automatically resetting torque limiter which is not excessively noisy when running in its released condition at high speeds.